In the digital printing of color images, image-based digital data causes placement of different colorants at specific locations on a print sheet. These colorants, such as toner or liquid ink, typically relate to primary colors which are blended together on the print sheet to yield a realistic image. Because a typical set of colorants would be cyan, magenta, and yellow (CMY), it follows that the image data, on a pixel-by-pixel basis, would be in a CMY color space.
Signals for television or video tape transmission, however, generally do not operate in a CMY or similar color space. Rather, there are provided separate luminance and chrominance signals. The luminance signal controls the overall brightness of a particular portion of an image being rendered, and one or more chrominance signals are used to indicate coloration of a particular area of the image. In the color signal specified by the National Television System Committee (NTSC), the chrominance signal is superimposed on a high-frequency region of the luminance signal; in general the purpose of this arrangement is to allow color original signals to be received on a black-and-white receiver.
When deriving still images from an original video source, such as for digital printing, another common image defect is a lack of alignment among pixels in the horizontal columns of the image. FIG. 1 illustrates this type of image defect in the case of a test image of a letter R which is disposed near an edge E of a sample image. It will be noticed that the first pixel of each successive horizontal line at edge E is not vertically aligned with its neighbors. Within the context of video images, this lack of alignment will cause a general "wobble" or "jitter" artifact in the image. This lack of vertical alignment is particularly noticeable with the use of consumer video cassette recorders. The lack of vertical alignment may not be objectionable in a moving image, but can be readily apparent when a single frame of the video image is sampled for printing. It is thus an object of another aspect of the present invention to provide a system whereby the print defect of lack of vertical alignment can be detected and then corrected.